1. Field of the Invention
The present invention relates to an improved process for beneficiating titaniferous materials whereby formation of fine particles during leaching of the titaniferous material is minimized.
2. Description of the Prior Art
Iron-containing titaniferous materials, such as ilmenite and leucoxene, and other titanium minerals and compositions are used in the production of titanium dioxide. These materials are used in the production of both pigmentary titanium dioxide and titanium dioxide manufactured for conversion into titanium metal. The titaniferous materials can be treated by the so-called "sulfate process" or by the "chloride process" which employs fluidized bed chlorination followed by vapor phase oxidation to form titanium dioxide. To conserve chlorine during the chlorination process, the titaniferous material preferably is beneficiated to enrich the titanium values and remove undesirable contaminants such as the iron.
Various methods such as magnetic separation and acidic leaching have been used to separate contaminants from the titaniferous material. In general, these methods are difficult to carry out, are uneconomical or produce undesirable by-products which are difficult to remove from the equipment or present disposal problems.
In the process of beneficiating a titaniferous material such as ilmenite by leaching with a mineral acid, the objective is to remove as much of the iron value and other impurities as possible while leaving the titanium values with a particle size most suited for processing into a pigment or titanium metal.
In reality, depending upon the titaniferous material, mineral acid and process conditions employed, a substantial portion of the titanium values are dissolved by the mineral acid. Then, under the influence of temperature and mineral acid concentration, as well as other factors, the dissolved titanium values hydrolyze to form titanium oxides that precipitate from the acidic solution. Some of the precipitated titanium oxide hydrates comprise "fines" or a "slime" of very small particle sizes. The fines are very difficult to separate and economically recover from the acidic solution. Further, if hydrochloric acid is used as the mineral acid and the fines are not removed from the acidic solution, the fines contaminate the iron oxide by-product produced during regeneration of the hydrochloric acid for recycle in the leaching process. The contaminating fines make the iron oxide undesirable for some uses, such as steel manufacture. Thus, fines formation is a direct loss of titanium value from the titaniferous material as well as an operational nuisance.
Various methods have been disclosed to minimize fines formation during acidic leaching of titaniferous materials. U.S. Pat. No. 3,193,376 discloses a two stage leaching process. In the first stage, the titaniferous material is contacted with a mixed acid solution of second stage leach liquor and sufficient fresh hydrochloric acid of at least 20.degree. Be. strength to provide about 60 percent of the stoichiometric amount of acid required to dissolve the impurities in the material. Patentees disclose that this solution preferentially dissolves the acid-soluble impurities while minimizing dissolution of titaniferous values in the first stage apparatus. The leached residue of the first stage then is leached in a second stage operation by contact with fresh hydrochloric acid of at least 20.degree. Be. strength. The fresh acid is present in an amount sufficient to provide about a 50 percent excess over the stoichiometric amount of acid required to dissolve the remaining acid-soluble impurities. The leached residue then is separated from the second stage leach liquor and recovered as a product. At least a portion of the second stage leach liquor is used as a part of the first stage leaching solution. The temperature of the first and second stage leach liquors is maintained above about 105.degree. C. during the major portion of the leaching operations.
U.S. Pat. No. 3,660,078 discloses a process for leaching a titaniferous material with a mineral acid in the presence of a seed material capable of accelerating the hydrolysis of dissolved titanium values. The seed material can comprise hydrated metal oxides of metals such as titanium, tin, niobium, tantalum, silicon and the like. The seed material generally is employed in a colloidal state. The leaching process is performed in a conventional manner with the exception that a seed material is present in the leaching system. The molar percentage of metal oxide in the seed material generally is an amount of from 0.1 to 10 percent as compared with the titanium dioxide in the titaniferous material. The titanium value that is dissolved is believed to precipitate upon the surface of the original feed material.
U.S. Pat. No. 4,019,898 discloses a process for beneficiating ilmenite by hydrochloric acid leaching in the presence of added sulfate ion. The sulfate ion is present in the leach liquor in an amount of from about 0.5 to 2.5 percent by weight of the ilmenite ore that is to be leached. The presence of the sulfate ion has been found to minimize fines formation and improve the color of the beneficiated ilmenite product.
While the prior art methods have provided some reduction in the quantity of fines produced during acid leaching of titaniferous materials, it still is desirable to provide improved methods that further reduce the quantity of fines that are produced.